1. Field of the Invention
The present invention relates to a computer system, its peripheral device, a computer, a control method thereof and a storage medium. More concretely, the present invention relates to a computer system which has a low power consumption state and a computer peripheral device capable of freely managing communication connection with a host computer, its peripheral device, a computer, a control method and a storage medium.
2. Related Background Art
In recent years, a method based on USB (Universal Serial Bus) was put to practical use as a method to connect peripheral devices with a personal computer (PC).
The USB which is the standard for connecting the personal computer with the plural peripheral devices in serial communication has Plug and Play (a function for automatically recognizing connection relations when the peripheral device is newly connected and disconnected), Hot Insertion (a function capable of connecting and disconnecting the peripheral device with a power supply on) or a hot-line insertion/separation function, and a power supply function. Thus, the USB is designed such that a user is never suffered for setting of addresses and ID numbers when he connects the peripheral devices with the personal computer. In the USB, four lines (a 5V power supply line called a VBUS line, a ground line called a GND line, a D+ signal line and a D− signal line) in total are detachably connected between the personal computer and the peripheral device by using a dedicated connector. There is a limit in a current value which can be supplied in the power supply line. Namely, on the USB standard, such the current value is limited to 100 mA and 500 mA.
Further, it is defined in the USB that the peripheral device must enter a suspend state by an instruction of a host computer. In this suspend state, it is necessary to reduce current consumption from the VBUS line with 500 μA. Incidentally, the peripheral device is restored by a resuming instruction.
The following problems are in the conventional USB peripheral devices.
Namely, regardless of the USB, when the power is supplied from the host computer side, naturally there is a limitation in the supplied current. Therefore, a high-speed circuit or machine which flows a large current exceeding the limitation value of the supply current cannot be installed in the peripheral device. In a digital still camera, a large current is temporarily necessary to charge an electronic flash and force back a shutter spring. However, due to the above limitation, the digital still camera could not be used as the USB peripheral device, especially as the USB peripheral device to which power is supplied from the host computer.
When a suspend instruction is issued from the host computer, the USB peripheral device must promptly suppress the current from the power supply. For example, when the power supply is intercepted while data is being written on a memory card, in the worst case there is a problem that the data in the memory card is destroyed. Therefore, it is difficult to use as the peripheral device a device such as an electronic still camera which writes some data, especially a large amount of data, on a memory card.
In a case where the peripheral device itself has a power supply, when an equipment required to carry is such the peripheral device, it has a built-in battery. However, especially in the USB, a communication circuit on the side of the peripheral device must always operate while the peripheral device is connected with the personal computer. This is because, since the host computer always generates a packet signal, this communication circuit must judge whether the generated packet signal is for this circuit itself and must respond to the host computer within a predetermined time if judged that the generated packet signal is for this circuit itself. Therefore, if the battery-driven device is connected with the USB as the USB peripheral device, an operation mode can not be shifted to a power saving mode, whereby the battery is consumed in a short time.
Further, in the USB, the host computer can recognize that the a USB device was removed, due to a decrease in a voltage level of the D+ line. When it is judged that the USB device is removed from a USB bus, a device driver (software) corresponding to the removed device is unloaded from a memory. Thus, only while the USB device is connected with the USB bus, the corresponding device driver (software) can be used on the host computer.
On the other hand, regardless of the USB, various kinds of devices which are connected with the personal computer and used conventionally exist. In some of these devices, for the purpose of power saving, an operation state is changed to a power saving mode or a power-off state when there is no access for a certain time.
Further, a digital camera which turns off a power supply when a state that any operation is not received continues for a certain time has been actually manufactured.
In the conventional USB, when the power supply of the USB device is turned off for power saving, also feeding to a pull-up resistor of the D+ line of the USB cable stops, whereby the voltage level of the D+ line decreases. As a result, though this USB device is connected with the host computer via the USB cable, the host computer judges that this USB device was removed and thus unloads the device driver (software) corresponding to this device, whereby this USB device can not be used. Namely, for a user of this peripheral device, a nonunderstandable situation that this USB device can not be used suddenly without any warning occurs.
Further, like another connection means, even if the device puts out some warning on a host computer for a user before the device comes to be in a power saving mode, there is the following problem.
Unlike a conventional RS-232C connection device, the USB device must be in a state capable of performing communication at the time when this device is connected with the host computer because of the following reason. Namely, in case of the RS-232C device, it is convenient that the host computer can communicate with the device only when the device is used. However, in case of the USB device, it is necessary to be able to transmit and receive a USB request signal necessary to recognize and structure the device at the time when the device is connected.
In a state that application software which can understand a message sent from the device does not start, when any operation to the device is not performed for a certain time, like the RS-232C connection device the message is sent to the application software to cause this software to display a warning to the user. However, at this time, since the application software which manages the USB device does not start yet, it is impossible to display the warning to the user.